When a golf club head strikes a golf ball slightly off center, that is, not directly in line with the center of gravity of the head, a spin is induced in the ball causing the ball to curve in flight. This effect is known as the gear effect and is thoroughly explored in U.S. Pat. No. 4,471,961 to Masghati et al. In essence, the off center impact of the ball causes the head to rotate about its center of gravity and this rotation, or spin, is transferred into the golf ball, like two gears meshing, so that the ball spins in the opposite direction. Spinning balls curve as they fly through the air.
In order to compensate for this adverse spin, golf club heads have convex curved impact surfaces, known as bulge, that intentionally deflect the ball sideways by increasing amounts for progressively further off center hits so that the inevitable curve will bring the ball roughly back to the desired path in line with the direction of the club swing. This compensation is approximate at best, and many prior art solutions have been attempted to minimize the gear effect.
The Masghati et al patent, for example, proposes weights in the head at locations far from the rotational axis so as to maximize the rotational inertia and, thus, resist head rotation. But trying to diminish head rotation in this manner is inherently limited because head weights are established at about 200 grams and even if nearly all of this weight is near the edge, the head will still twist in response to off center ball strikes.
Another problem in maintaining accuracy with convex impact surfaces is that all materials have some degree of elasticity. Hence, no matter what material a golf club head is made from, it will elastically deform from the impact of the ball so that the actual shape of the bulge is never fully predictable. It follows that the intentional misdirection of the ball, to compensate for spin, is also somewhat unpredictable. This temporary shape change may also affect the efficiency of the spin transfer to the ball, and, consequently, the magnitude of the gear effect. Modern club heads are typically constructed from a thin metal shell that is filled with a fairly stiff plastic foam. This produces a very durable, and very strong, head with a naturally high rotational moment of inertia. However, the head is still a highly dynamically flexible object which must necessarily yield and bend a bit upon impact. If it did not yield, that is, was very hard, it would be, by definition, brittle and would likely shatter on impact.
The discussion above concerns the technology surrounding the very long driving of balls and thus pertains more to drivers and the first shot off a well prepared tee. Once on the fairway, other problems arise as well, although distance may still be required. In this situation, golfers often switch from a driver to a so called wood, which may, in modern practice, be made from metal, wood, or plastics. To facilitate use of the club in taller grass, or in the rough, another class of prior art designs has emerged that use skids or strakes on the bottom surface, or sole, of the club head. These skids slide over or through the ground and keep the bulk of the bottom surface of the head from digging into the ground and grass and, thus, losing momentum. Typical prior art in this class is found in U.S. Pat. Nos. 4,332,388 to Crow, 5,213,329 to Okumoto et al, 5,125,662 to Antonious, 3,761,095 to Thompson, and 3,815,921 to Turner. All of these patents show various kinds of skids or runners on the bottom of the head intended to help the head track smoothly and more easily over the surface of the ground. All of these patents also require that the skids be parallel to the direction of movement of the head or swing. Crow, for example, says in column 3, line 32, that his parallel runners stabilize and track the head when it contacts the ground. Okumoto states that his skids extend in a back and forth direction relative to the direction of swing of the club. Antonious says in column 2, line 6, that his skid members have their longitudinal axis extending in the front to rear direction so as to furrow into the ground and prevent lateral club head movement (column 2, line 53). Thomson recites in column 2, that his so called keel 30 extends along a line that will be the path of swing of the head. Finally, Turner says that his keel is normal to the striking face in column 1, line 36. Clearly, it is the firm opinion in the art that strakes on the club head must be parallel to the swing path to avoid sideways deflections of the club upon ground contact. The present invention contemplates a complete reversal of this orthodoxy in order to achieve not only the benefits of skids, but also the taming of the gear effect, as explained below.